| use crate::infer::InferCtxt; |
| use crate::opaque_types::required_region_bounds; |
| use crate::traits; |
| use rustc_hir as hir; |
| use rustc_hir::def_id::DefId; |
| use rustc_hir::lang_items; |
| use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef}; |
| use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness}; |
| use rustc_span::Span; |
| use std::rc::Rc; |
| |
| /// Returns the set of obligations needed to make `arg` well-formed. |
| /// If `arg` contains unresolved inference variables, this may include |
| /// further WF obligations. However, if `arg` IS an unresolved |
| /// inference variable, returns `None`, because we are not able to |
| /// make any progress at all. This is to prevent "livelock" where we |
| /// say "$0 is WF if $0 is WF". |
| pub fn obligations<'a, 'tcx>( |
| infcx: &InferCtxt<'a, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| body_id: hir::HirId, |
| arg: GenericArg<'tcx>, |
| span: Span, |
| ) -> Option<Vec<traits::PredicateObligation<'tcx>>> { |
| // Handle the "livelock" case (see comment above) by bailing out if necessary. |
| let arg = match arg.unpack() { |
| GenericArgKind::Type(ty) => { |
| match ty.kind { |
| ty::Infer(ty::TyVar(_)) => { |
| let resolved_ty = infcx.shallow_resolve(ty); |
| if resolved_ty == ty { |
| // No progress, bail out to prevent "livelock". |
| return None; |
| } |
| |
| resolved_ty |
| } |
| _ => ty, |
| } |
| .into() |
| } |
| GenericArgKind::Const(ct) => { |
| match ct.val { |
| ty::ConstKind::Infer(infer) => { |
| let resolved = infcx.shallow_resolve(infer); |
| if resolved == infer { |
| // No progress. |
| return None; |
| } |
| |
| infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Infer(resolved), ty: ct.ty }) |
| } |
| _ => ct, |
| } |
| .into() |
| } |
| // There is nothing we have to do for lifetimes. |
| GenericArgKind::Lifetime(..) => return Some(Vec::new()), |
| }; |
| |
| let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None }; |
| wf.compute(arg); |
| debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out); |
| |
| let result = wf.normalize(); |
| debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result); |
| Some(result) |
| } |
| |
| /// Returns the obligations that make this trait reference |
| /// well-formed. For example, if there is a trait `Set` defined like |
| /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF |
| /// if `Bar: Eq`. |
| pub fn trait_obligations<'a, 'tcx>( |
| infcx: &InferCtxt<'a, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| body_id: hir::HirId, |
| trait_ref: &ty::TraitRef<'tcx>, |
| span: Span, |
| item: Option<&'tcx hir::Item<'tcx>>, |
| ) -> Vec<traits::PredicateObligation<'tcx>> { |
| let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item }; |
| wf.compute_trait_ref(trait_ref, Elaborate::All); |
| wf.normalize() |
| } |
| |
| pub fn predicate_obligations<'a, 'tcx>( |
| infcx: &InferCtxt<'a, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| body_id: hir::HirId, |
| predicate: ty::Predicate<'tcx>, |
| span: Span, |
| ) -> Vec<traits::PredicateObligation<'tcx>> { |
| let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None }; |
| |
| // (*) ok to skip binders, because wf code is prepared for it |
| match predicate.kind() { |
| ty::PredicateKind::Trait(t, _) => { |
| wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*) |
| } |
| ty::PredicateKind::RegionOutlives(..) => {} |
| ty::PredicateKind::TypeOutlives(t) => { |
| wf.compute(t.skip_binder().0.into()); |
| } |
| ty::PredicateKind::Projection(t) => { |
| let t = t.skip_binder(); // (*) |
| wf.compute_projection(t.projection_ty); |
| wf.compute(t.ty.into()); |
| } |
| &ty::PredicateKind::WellFormed(arg) => { |
| wf.compute(arg); |
| } |
| ty::PredicateKind::ObjectSafe(_) => {} |
| ty::PredicateKind::ClosureKind(..) => {} |
| ty::PredicateKind::Subtype(data) => { |
| wf.compute(data.skip_binder().a.into()); // (*) |
| wf.compute(data.skip_binder().b.into()); // (*) |
| } |
| &ty::PredicateKind::ConstEvaluatable(def_id, substs) => { |
| let obligations = wf.nominal_obligations(def_id, substs); |
| wf.out.extend(obligations); |
| |
| for arg in substs.iter() { |
| wf.compute(arg); |
| } |
| } |
| &ty::PredicateKind::ConstEquate(c1, c2) => { |
| wf.compute(c1.into()); |
| wf.compute(c2.into()); |
| } |
| } |
| |
| wf.normalize() |
| } |
| |
| struct WfPredicates<'a, 'tcx> { |
| infcx: &'a InferCtxt<'a, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| body_id: hir::HirId, |
| span: Span, |
| out: Vec<traits::PredicateObligation<'tcx>>, |
| item: Option<&'tcx hir::Item<'tcx>>, |
| } |
| |
| /// Controls whether we "elaborate" supertraits and so forth on the WF |
| /// predicates. This is a kind of hack to address #43784. The |
| /// underlying problem in that issue was a trait structure like: |
| /// |
| /// ``` |
| /// trait Foo: Copy { } |
| /// trait Bar: Foo { } |
| /// impl<T: Bar> Foo for T { } |
| /// impl<T> Bar for T { } |
| /// ``` |
| /// |
| /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but |
| /// we decide that this is true because `T: Bar` is in the |
| /// where-clauses (and we can elaborate that to include `T: |
| /// Copy`). This wouldn't be a problem, except that when we check the |
| /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo` |
| /// impl. And so nowhere did we check that `T: Copy` holds! |
| /// |
| /// To resolve this, we elaborate the WF requirements that must be |
| /// proven when checking impls. This means that (e.g.) the `impl Bar |
| /// for T` will be forced to prove not only that `T: Foo` but also `T: |
| /// Copy` (which it won't be able to do, because there is no `Copy` |
| /// impl for `T`). |
| #[derive(Debug, PartialEq, Eq, Copy, Clone)] |
| enum Elaborate { |
| All, |
| None, |
| } |
| |
| fn extend_cause_with_original_assoc_item_obligation<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| trait_ref: &ty::TraitRef<'tcx>, |
| item: Option<&hir::Item<'tcx>>, |
| cause: &mut traits::ObligationCause<'tcx>, |
| pred: &ty::Predicate<'_>, |
| mut trait_assoc_items: impl Iterator<Item = &'tcx ty::AssocItem>, |
| ) { |
| debug!( |
| "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}", |
| trait_ref, item, cause, pred |
| ); |
| let items = match item { |
| Some(hir::Item { kind: hir::ItemKind::Impl { items, .. }, .. }) => items, |
| _ => return, |
| }; |
| let fix_span = |
| |impl_item_ref: &hir::ImplItemRef<'_>| match tcx.hir().impl_item(impl_item_ref.id).kind { |
| hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span, |
| _ => impl_item_ref.span, |
| }; |
| match pred.kind() { |
| ty::PredicateKind::Projection(proj) => { |
| // The obligation comes not from the current `impl` nor the `trait` being implemented, |
| // but rather from a "second order" obligation, where an associated type has a |
| // projection coming from another associated type. See |
| // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and |
| // `traits-assoc-type-in-supertrait-bad.rs`. |
| let kind = &proj.ty().skip_binder().kind; |
| if let ty::Projection(projection_ty) = kind { |
| let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id); |
| if let Some(impl_item_span) = |
| items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span) |
| { |
| cause.make_mut().span = impl_item_span; |
| } |
| } |
| } |
| ty::PredicateKind::Trait(pred, _) => { |
| // An associated item obligation born out of the `trait` failed to be met. An example |
| // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`. |
| debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred); |
| if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = |
| &pred.skip_binder().self_ty().kind |
| { |
| if let Some(impl_item_span) = trait_assoc_items |
| .find(|i| i.def_id == *item_def_id) |
| .and_then(|trait_assoc_item| { |
| items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span) |
| }) |
| { |
| cause.make_mut().span = impl_item_span; |
| } |
| } |
| } |
| _ => {} |
| } |
| } |
| |
| impl<'a, 'tcx> WfPredicates<'a, 'tcx> { |
| fn tcx(&self) -> TyCtxt<'tcx> { |
| self.infcx.tcx |
| } |
| |
| fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> { |
| traits::ObligationCause::new(self.span, self.body_id, code) |
| } |
| |
| fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> { |
| let cause = self.cause(traits::MiscObligation); |
| let infcx = &mut self.infcx; |
| let param_env = self.param_env; |
| let mut obligations = Vec::with_capacity(self.out.len()); |
| for pred in &self.out { |
| assert!(!pred.has_escaping_bound_vars()); |
| let mut selcx = traits::SelectionContext::new(infcx); |
| let i = obligations.len(); |
| let value = |
| traits::normalize_to(&mut selcx, param_env, cause.clone(), pred, &mut obligations); |
| obligations.insert(i, value); |
| } |
| obligations |
| } |
| |
| /// Pushes the obligations required for `trait_ref` to be WF into `self.out`. |
| fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) { |
| let tcx = self.infcx.tcx; |
| let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs); |
| |
| debug!("compute_trait_ref obligations {:?}", obligations); |
| let cause = self.cause(traits::MiscObligation); |
| let param_env = self.param_env; |
| |
| let item = self.item; |
| |
| let extend = |obligation: traits::PredicateObligation<'tcx>| { |
| let mut cause = cause.clone(); |
| if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() { |
| let derived_cause = traits::DerivedObligationCause { |
| parent_trait_ref, |
| parent_code: Rc::new(obligation.cause.code.clone()), |
| }; |
| cause.make_mut().code = |
| traits::ObligationCauseCode::DerivedObligation(derived_cause); |
| } |
| extend_cause_with_original_assoc_item_obligation( |
| tcx, |
| trait_ref, |
| item, |
| &mut cause, |
| &obligation.predicate, |
| tcx.associated_items(trait_ref.def_id).in_definition_order(), |
| ); |
| traits::Obligation::new(cause, param_env, obligation.predicate) |
| }; |
| |
| if let Elaborate::All = elaborate { |
| let implied_obligations = traits::util::elaborate_obligations(tcx, obligations); |
| let implied_obligations = implied_obligations.map(extend); |
| self.out.extend(implied_obligations); |
| } else { |
| self.out.extend(obligations); |
| } |
| |
| let tcx = self.tcx(); |
| self.out.extend( |
| trait_ref |
| .substs |
| .iter() |
| .filter(|arg| { |
| matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..)) |
| }) |
| .filter(|arg| !arg.has_escaping_bound_vars()) |
| .map(|arg| { |
| traits::Obligation::new( |
| cause.clone(), |
| param_env, |
| ty::PredicateKind::WellFormed(arg).to_predicate(tcx), |
| ) |
| }), |
| ); |
| } |
| |
| /// Pushes the obligations required for `trait_ref::Item` to be WF |
| /// into `self.out`. |
| fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) { |
| // A projection is well-formed if (a) the trait ref itself is |
| // WF and (b) the trait-ref holds. (It may also be |
| // normalizable and be WF that way.) |
| let trait_ref = data.trait_ref(self.infcx.tcx); |
| self.compute_trait_ref(&trait_ref, Elaborate::None); |
| |
| if !data.has_escaping_bound_vars() { |
| let predicate = trait_ref.without_const().to_predicate(self.infcx.tcx); |
| let cause = self.cause(traits::ProjectionWf(data)); |
| self.out.push(traits::Obligation::new(cause, self.param_env, predicate)); |
| } |
| } |
| |
| fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) { |
| if !subty.has_escaping_bound_vars() { |
| let cause = self.cause(cause); |
| let trait_ref = ty::TraitRef { |
| def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None), |
| substs: self.infcx.tcx.mk_substs_trait(subty, &[]), |
| }; |
| self.out.push(traits::Obligation::new( |
| cause, |
| self.param_env, |
| trait_ref.without_const().to_predicate(self.infcx.tcx), |
| )); |
| } |
| } |
| |
| /// Pushes all the predicates needed to validate that `ty` is WF into `out`. |
| fn compute(&mut self, arg: GenericArg<'tcx>) { |
| let mut walker = arg.walk(); |
| let param_env = self.param_env; |
| while let Some(arg) = walker.next() { |
| let ty = match arg.unpack() { |
| GenericArgKind::Type(ty) => ty, |
| |
| // No WF constraints for lifetimes being present, any outlives |
| // obligations are handled by the parent (e.g. `ty::Ref`). |
| GenericArgKind::Lifetime(_) => continue, |
| |
| GenericArgKind::Const(constant) => { |
| match constant.val { |
| ty::ConstKind::Unevaluated(def_id, substs, promoted) => { |
| assert!(promoted.is_none()); |
| |
| let obligations = self.nominal_obligations(def_id, substs); |
| self.out.extend(obligations); |
| |
| let predicate = ty::PredicateKind::ConstEvaluatable(def_id, substs) |
| .to_predicate(self.tcx()); |
| let cause = self.cause(traits::MiscObligation); |
| self.out.push(traits::Obligation::new( |
| cause, |
| self.param_env, |
| predicate, |
| )); |
| } |
| ty::ConstKind::Infer(infer) => { |
| let resolved = self.infcx.shallow_resolve(infer); |
| // the `InferConst` changed, meaning that we made progress. |
| if resolved != infer { |
| let cause = self.cause(traits::MiscObligation); |
| |
| let resolved_constant = self.infcx.tcx.mk_const(ty::Const { |
| val: ty::ConstKind::Infer(resolved), |
| ..*constant |
| }); |
| self.out.push(traits::Obligation::new( |
| cause, |
| self.param_env, |
| ty::PredicateKind::WellFormed(resolved_constant.into()) |
| .to_predicate(self.tcx()), |
| )); |
| } |
| } |
| ty::ConstKind::Error(_) |
| | ty::ConstKind::Param(_) |
| | ty::ConstKind::Bound(..) |
| | ty::ConstKind::Placeholder(..) => { |
| // These variants are trivially WF, so nothing to do here. |
| } |
| ty::ConstKind::Value(..) => { |
| // FIXME: Enforce that values are structurally-matchable. |
| } |
| } |
| continue; |
| } |
| }; |
| |
| match ty.kind { |
| ty::Bool |
| | ty::Char |
| | ty::Int(..) |
| | ty::Uint(..) |
| | ty::Float(..) |
| | ty::Error(_) |
| | ty::Str |
| | ty::GeneratorWitness(..) |
| | ty::Never |
| | ty::Param(_) |
| | ty::Bound(..) |
| | ty::Placeholder(..) |
| | ty::Foreign(..) => { |
| // WfScalar, WfParameter, etc |
| } |
| |
| // Can only infer to `ty::Int(_) | ty::Uint(_)`. |
| ty::Infer(ty::IntVar(_)) => {} |
| |
| // Can only infer to `ty::Float(_)`. |
| ty::Infer(ty::FloatVar(_)) => {} |
| |
| ty::Slice(subty) => { |
| self.require_sized(subty, traits::SliceOrArrayElem); |
| } |
| |
| ty::Array(subty, _) => { |
| self.require_sized(subty, traits::SliceOrArrayElem); |
| // Note that we handle the len is implicitly checked while walking `arg`. |
| } |
| |
| ty::Tuple(ref tys) => { |
| if let Some((_last, rest)) = tys.split_last() { |
| for elem in rest { |
| self.require_sized(elem.expect_ty(), traits::TupleElem); |
| } |
| } |
| } |
| |
| ty::RawPtr(_) => { |
| // Simple cases that are WF if their type args are WF. |
| } |
| |
| ty::Projection(data) => { |
| walker.skip_current_subtree(); // Subtree handled by compute_projection. |
| self.compute_projection(data); |
| } |
| |
| ty::Adt(def, substs) => { |
| // WfNominalType |
| let obligations = self.nominal_obligations(def.did, substs); |
| self.out.extend(obligations); |
| } |
| |
| ty::FnDef(did, substs) => { |
| let obligations = self.nominal_obligations(did, substs); |
| self.out.extend(obligations); |
| } |
| |
| ty::Ref(r, rty, _) => { |
| // WfReference |
| if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() { |
| let cause = self.cause(traits::ReferenceOutlivesReferent(ty)); |
| self.out.push(traits::Obligation::new( |
| cause, |
| param_env, |
| ty::PredicateKind::TypeOutlives(ty::Binder::dummy( |
| ty::OutlivesPredicate(rty, r), |
| )) |
| .to_predicate(self.tcx()), |
| )); |
| } |
| } |
| |
| ty::Generator(..) => { |
| // Walk ALL the types in the generator: this will |
| // include the upvar types as well as the yield |
| // type. Note that this is mildly distinct from |
| // the closure case, where we have to be careful |
| // about the signature of the closure. We don't |
| // have the problem of implied bounds here since |
| // generators don't take arguments. |
| } |
| |
| ty::Closure(_, substs) => { |
| // Only check the upvar types for WF, not the rest |
| // of the types within. This is needed because we |
| // capture the signature and it may not be WF |
| // without the implied bounds. Consider a closure |
| // like `|x: &'a T|` -- it may be that `T: 'a` is |
| // not known to hold in the creator's context (and |
| // indeed the closure may not be invoked by its |
| // creator, but rather turned to someone who *can* |
| // verify that). |
| // |
| // The special treatment of closures here really |
| // ought not to be necessary either; the problem |
| // is related to #25860 -- there is no way for us |
| // to express a fn type complete with the implied |
| // bounds that it is assuming. I think in reality |
| // the WF rules around fn are a bit messed up, and |
| // that is the rot problem: `fn(&'a T)` should |
| // probably always be WF, because it should be |
| // shorthand for something like `where(T: 'a) { |
| // fn(&'a T) }`, as discussed in #25860. |
| // |
| // Note that we are also skipping the generic |
| // types. This is consistent with the `outlives` |
| // code, but anyway doesn't matter: within the fn |
| // body where they are created, the generics will |
| // always be WF, and outside of that fn body we |
| // are not directly inspecting closure types |
| // anyway, except via auto trait matching (which |
| // only inspects the upvar types). |
| walker.skip_current_subtree(); // subtree handled below |
| for upvar_ty in substs.as_closure().upvar_tys() { |
| // FIXME(eddyb) add the type to `walker` instead of recursing. |
| self.compute(upvar_ty.into()); |
| } |
| } |
| |
| ty::FnPtr(_) => { |
| // let the loop iterate into the argument/return |
| // types appearing in the fn signature |
| } |
| |
| ty::Opaque(did, substs) => { |
| // all of the requirements on type parameters |
| // should've been checked by the instantiation |
| // of whatever returned this exact `impl Trait`. |
| |
| // for named opaque `impl Trait` types we still need to check them |
| if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() { |
| let obligations = self.nominal_obligations(did, substs); |
| self.out.extend(obligations); |
| } |
| } |
| |
| ty::Dynamic(data, r) => { |
| // WfObject |
| // |
| // Here, we defer WF checking due to higher-ranked |
| // regions. This is perhaps not ideal. |
| self.from_object_ty(ty, data, r); |
| |
| // FIXME(#27579) RFC also considers adding trait |
| // obligations that don't refer to Self and |
| // checking those |
| |
| let defer_to_coercion = self.tcx().features().object_safe_for_dispatch; |
| |
| if !defer_to_coercion { |
| let cause = self.cause(traits::MiscObligation); |
| let component_traits = data.auto_traits().chain(data.principal_def_id()); |
| let tcx = self.tcx(); |
| self.out.extend(component_traits.map(|did| { |
| traits::Obligation::new( |
| cause.clone(), |
| param_env, |
| ty::PredicateKind::ObjectSafe(did).to_predicate(tcx), |
| ) |
| })); |
| } |
| } |
| |
| // Inference variables are the complicated case, since we don't |
| // know what type they are. We do two things: |
| // |
| // 1. Check if they have been resolved, and if so proceed with |
| // THAT type. |
| // 2. If not, we've at least simplified things (e.g., we went |
| // from `Vec<$0>: WF` to `$0: WF`), so we can |
| // register a pending obligation and keep |
| // moving. (Goal is that an "inductive hypothesis" |
| // is satisfied to ensure termination.) |
| // See also the comment on `fn obligations`, describing "livelock" |
| // prevention, which happens before this can be reached. |
| ty::Infer(_) => { |
| let ty = self.infcx.shallow_resolve(ty); |
| if let ty::Infer(ty::TyVar(_)) = ty.kind { |
| // Not yet resolved, but we've made progress. |
| let cause = self.cause(traits::MiscObligation); |
| self.out.push(traits::Obligation::new( |
| cause, |
| param_env, |
| ty::PredicateKind::WellFormed(ty.into()).to_predicate(self.tcx()), |
| )); |
| } else { |
| // Yes, resolved, proceed with the result. |
| // FIXME(eddyb) add the type to `walker` instead of recursing. |
| self.compute(ty.into()); |
| } |
| } |
| } |
| } |
| } |
| |
| fn nominal_obligations( |
| &mut self, |
| def_id: DefId, |
| substs: SubstsRef<'tcx>, |
| ) -> Vec<traits::PredicateObligation<'tcx>> { |
| let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs); |
| predicates |
| .predicates |
| .into_iter() |
| .zip(predicates.spans.into_iter()) |
| .map(|(pred, span)| { |
| let cause = self.cause(traits::BindingObligation(def_id, span)); |
| traits::Obligation::new(cause, self.param_env, pred) |
| }) |
| .filter(|pred| !pred.has_escaping_bound_vars()) |
| .collect() |
| } |
| |
| fn from_object_ty( |
| &mut self, |
| ty: Ty<'tcx>, |
| data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>, |
| region: ty::Region<'tcx>, |
| ) { |
| // Imagine a type like this: |
| // |
| // trait Foo { } |
| // trait Bar<'c> : 'c { } |
| // |
| // &'b (Foo+'c+Bar<'d>) |
| // ^ |
| // |
| // In this case, the following relationships must hold: |
| // |
| // 'b <= 'c |
| // 'd <= 'c |
| // |
| // The first conditions is due to the normal region pointer |
| // rules, which say that a reference cannot outlive its |
| // referent. |
| // |
| // The final condition may be a bit surprising. In particular, |
| // you may expect that it would have been `'c <= 'd`, since |
| // usually lifetimes of outer things are conservative |
| // approximations for inner things. However, it works somewhat |
| // differently with trait objects: here the idea is that if the |
| // user specifies a region bound (`'c`, in this case) it is the |
| // "master bound" that *implies* that bounds from other traits are |
| // all met. (Remember that *all bounds* in a type like |
| // `Foo+Bar+Zed` must be met, not just one, hence if we write |
| // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and |
| // 'y.) |
| // |
| // Note: in fact we only permit builtin traits, not `Bar<'d>`, I |
| // am looking forward to the future here. |
| if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() { |
| let implicit_bounds = object_region_bounds(self.infcx.tcx, data); |
| |
| let explicit_bound = region; |
| |
| self.out.reserve(implicit_bounds.len()); |
| for implicit_bound in implicit_bounds { |
| let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound)); |
| let outlives = |
| ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound)); |
| self.out.push(traits::Obligation::new( |
| cause, |
| self.param_env, |
| outlives.to_predicate(self.infcx.tcx), |
| )); |
| } |
| } |
| } |
| } |
| |
| /// Given an object type like `SomeTrait + Send`, computes the lifetime |
| /// bounds that must hold on the elided self type. These are derived |
| /// from the declarations of `SomeTrait`, `Send`, and friends -- if |
| /// they declare `trait SomeTrait : 'static`, for example, then |
| /// `'static` would appear in the list. The hard work is done by |
| /// `infer::required_region_bounds`, see that for more information. |
| pub fn object_region_bounds<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>, |
| ) -> Vec<ty::Region<'tcx>> { |
| // Since we don't actually *know* the self type for an object, |
| // this "open(err)" serves as a kind of dummy standin -- basically |
| // a placeholder type. |
| let open_ty = tcx.mk_ty_infer(ty::FreshTy(0)); |
| |
| let predicates = existential_predicates.iter().filter_map(|predicate| { |
| if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() { |
| None |
| } else { |
| Some(predicate.with_self_ty(tcx, open_ty)) |
| } |
| }); |
| |
| required_region_bounds(tcx, open_ty, predicates) |
| } |
